1. Field of the Invention
This invention generally relates to optical lens positioning systems. More particularly, the invention relates to a system of focusing lenses with a linear actuator and position sensing means in a lightweight compact assembly.
2. Description of the Related Art
The most common point of sale device (POSD) used today is a laser barcode scanner. The system relies upon a machine readable label printed on most products by a plurality of manufacturers. The label is a series of parallel bars of varying widths, encoding numerical information known as the Universal Product Code (UPC). In operation, the device scans a focused laser beam back and forth across the label many times each second in a linear or intricate pattern. A detector, sensitive only to the laser wavelength, detects the variations in light reflected from the dark and light bands of the label and converts them into electrical signals. When the label is scanned, the reflected light energy is processed and the price of the identified item is posted.
The ability of a laser beam to deliver high power densities to a small area makes the laser attractive for this application. The high degree of transverse coherence permits the light from a laser to be focused onto a much smaller area than the light from a more conventional source.
Related to the spot size of the beam is the distance, and the region of the focus spot, over which the beam diameter remains small. This distance, known as the depth of field, is important in a scanning system, for determining where the object can be positioned without having to refocus. The depth of field is somewhat arbitrary in that it depends on the beam focus tolerance that one chooses to apply. However, a large depth of field is not compatible with a small focused spot size. Due to this limitation, to use a laser in an application requiring a large depth of field while requiring a small laser spot size necessitates refocusing the laser beam.
FIG. 1 shows a simple relationship of a light source 17 and a double convex or condenser lens 19. The converging rays exiting the lens 21a, 21b, 21c form a waste 23 and then diverge again 21c, 21b, 21a.
The laser beam used in a scanning device is optically modified usually by a lens containing more than one element to form a waste shaped beam in which the width of the beam diminishes with distance until reaching a minimum, or waste and then increases. The beam is focused by the last element of the combination lens so that its desired spot size or waste is achieved at the object or barcode distance.
As shown in FIG. 2, the most common method to increase the depth of field 25 of a lens 19 is to reduce its lens aperture 27 with a stop 29. Some of the rays 21a, 21c entering the lens are blocked by the stop 29. The converging rays exiting the lens 21d, 21b, 21e have been reduced but create a greater depth of field 25 located at the waste 23. The source 17 will remain focused in dependence upon the stop 29 and the focal length of the lens. However, by increasing or decreasing the stop 29 provides only a limited depth of field.
To better the range of focus beyond a preset depth of field requires refocusing the lens. One method to focus a projected beam is by using a stepper motor to drive a mechanism holding at least one element of the lens optics. However, the use of prior art stepper motors for lens positioning leaves much to be desired since stepper motors have a finite step angle which ultimately results in a series of finite positions.
Another method to provide the motive force to position a lens element is to use a magnetic coil or linear actuator, well known in the art. Similarly, the prior art attempts to use linear actuators resulted in increased mechanical complexity with awkward arrangements of linkages to position the optics and provide corrective feedback.
In the alternative, to displace the light source from the optics thereby achieving the same result would require an even more complicated mechanical approach. In order to decrease the likelihood of errors and thereby increasing the efficiency of laser barcode scanners, it is desirable to have a simple, accurate and inexpensive focusing means for laser scanners.